Improve emoji picker

This request uses an emoji image sprite to show big set of emojis. The sprite has been made based one gemojione gem images. The categories are retrieved from https://github.com/jonathanwiesel/gemojione/blob/master/config/index.json as well.

Some categories are pretty poor, it can be extended in the future.

It's a first step on the way to improve picker.

The first implementation was using https://github.com/iamcal/emoji-data project, but there was huge incompatibility with gemojione emojis set and I had to drop that solution.

![Screenshot_2015-12-22_23.26.27](/uploads/7d323eb0586204ac92915b41233b97ec/Screenshot_2015-12-22_23.26.27.png)


See merge request !2172